This read-write enumeration sets the size, in milliseconds (10, 20, 30, or 40ms), of the primary high-speed data packet transmitted after the first phase of a T.38 fax call. An admin adjusts this to control how the gateway chunks the high-speed image data phase of a fax transmission, trading packet count against packet size for the network conditions in use. For example, choosing tenms on a jittery link produces more, smaller high-speed packets so a single loss carries less fax image data.